At present, one of the major methods of treatment of infectious disease is the administration of chemotherapeutic agents, either synthetic (e.g., sulfonilamides) or natural (e.g. streptomycin). Generally speaking, the methods produce the desired affect of inhibition or destruction of the infectious agent by interfering with one or more of its metabolic pathways in some manner. Although certainly effective, the use of these substances may have a number of undesirable side effects. For example, many patients may experience sensitive or allergic reactions, kidney and gastrointestinal tract irritation, or nerve damage. This is in large part due to the homology between the metabolic pathways of the host and the invading organism. This is especially a problem in the case where both pathogen and host are eukaryote, such as in fungal diseases of man. Further, the use of antibiotics may promote development of resistant forms of the pathogens, a problem with serious long-term side effects extending beyond just the individual patient.
Alternatively, with a disease in which the metabolic pathways of the causative agent are unknown or poorly understood, oftentimes the symptoms alone are treated until the natural immunity of host can respond. In such a case the obvious danger is that considerable, possibly irreversible, damage may be done by the pathogen before the host's defenses have had sufficient time to combat the infection.
A method of treatment of infectious organisms which avoids these difficulties is clearly desirable. To be completely successful, the treatment should kill the invading organism while causing substantially no harm to the host's tissues. Also, the treatment should be such that there is little or no chance for the organisms being treated to become tolerant or resistant to the treatment method. Also desirable would be a method of treatment, which would permit the destruction of an organism about which only a minimum of information is known concerning its metabolic pathways. One potential method is to focus upon a particular aspect of the infectious organism's metabolism which differs from that of the host cells, and, rather than interfering with the pathways as antibiotics do, to exploit the organism's routine use of that pathway in such a way that it may be turned against the organism, eventually killing it.
An example of such a potentially useful pathway is that by which infectious organisms acquire and store iron. For convenience, the invention will be described in relation to iron metabolism, but as will be made clear below, the invention is not limited to embodiments relating to iron metabolism per se.
Numerous investigations have been conducted to determine the method of uptake of iron by microorganisms from their environment. It has been found that many groups of microbes differ in their mode of iron acquisition, and most appear to differ fundamentally in the pathways employed by the cells of higher organisms. For example, it is widely accepted that bacteria may produce chelating agents which have high affinity for certain metals, and particularly ferric iron. In all studied enteric bacteria, such as Salmonella, Enterobacter, Klebsiella, and Escherichia, this chelator is enterochelin, a cyclic trimer of 2,3-dihydroxybenzoylserine (Rosenberg and Young, Microb. Iron Metab., J. B. Nielands, ed., p. 67, 1974). The mycobacteria produce a series of secondary hydroxamates known as mycobactins, and also salicylic acid. Bacillus (sps) are known to utilize 2,3-dihydroxybenzoylglycine for iron transport (Byers, Microb. Iron Metab., J. B. Nielands, ed., p. 83, 1974). In most cases, these substances are excreted into the environment where they bind iron, and the entire iron-chelator complex is reabsorbed by the bacterium.
Iron transport mechanisms have been characterized in microorganisms other than bacteria as well. Ustilago sphaerogena, a smut fungus, utilizes the cyclic hexapeptide desferri-ferrichrome as its iron carrier (Emery, Microb. Iron Metab., J. B. Nielands, ed., p. 107, 1974). In protozoa, ingestion of ferric hydroxide particles by pinocytosis is probably the mechanism of iron uptake. Certain RNA viruses have also been shown to bind terbium to their nucleic acid (Morley, et al., Biochem. Biophys. Res. Comm., 101:1123, 1981).
It is also known that a wide variety of metal-containing molecules occur naturally with the microbial cell; the most familiar of these are the porphyrins, and especially important among these are the protoporphyrins, including the chlorophylls and cytochromes. However, many microorganisms are also known to contain specific iron-sulfur containing proteins, such as ferredoxin and rubredoxin, which serve as electron-transfer factors (Lovenberg, Microb. Iron Metab., J. B. Nielands, ed., p. 161, 1974). These proteins may be of different structures in the various organisms from which they have been isolated, but always consist of an iron-containing center which may consist of from one iron (rubredoxin) up to four irons (Clostridium ferredoxin).
Minute particles possessing ferromagnetic, paramagnetic or diamagnetic properties have been shown to be particularly useful in treating cancer, as described by R. T. Gordon in U.S. Pat. No. 4,106,488 and U.S. patent application Ser. No. 464,870, filed Feb. 8, 1983, incorporated herein by reference. As exemplified therein, ferric hydroxide and gallium citrate are used to form particles of a size 1 micron or less, and are introduced into the cancer cells in the area to be treated. The cells of the chosen area are then subjected to a high frequency alternating magnetic field, inductively heating the intracellular particles, resulting in an increase in intracellular temperature. Because the cancer cells accumulate the particles to a greater degree than normal cells, and because they also have a higher resting temperature than normal cells, the increase in temperature kills the cancer cells and leaves the normal cells substantially unharmed. The present invention is predicated on the discovery that with certain modification, the intracellular hyperthermia technique as disclosed by Gordon may be effectively utilized in destroying the cells of infectious organisms, exploiting the specificity of some of their metabolic pathways and metal-containing, metabolizable products to selectively concentrate the magnetic particles within the cells of the disease-causing organisms, or to selectively focus the inductive heating process upon magnetic particles found naturally only in the infectious cells.